this should simplify building SciTECO for new users
* compiler and archiver are passed down from Autoconf,
so cross-compiling should work transparently
* `make clean` will also clean the Scintilla source tree
* there is no longer any need for "source bundles" as
tar balls also contain Scintilla/Scinterm now
* building from Git is not much more difficult than building
from a tar ball
* The versions of Scintilla/Scinterm embedded as submodules
already contain all the patches necessary (currently none are
necessary), so there's no need to have patch files in the
repository
* INSTALL instructions have been rewritten
* the --with-scintilla and --with-scinterm site-config options
have been kept. But they should be rarely necessary now.